With fluorescence, if you can look at it yourself to judge that would be best. Fluorescence when strong, makes the diamond glow in natural light, even making the diamond seem hazy. If it is faint and worn on the neck I do not think faint will be an issue.
I also look at face up area vs carat weight too. To ensure the diamond isn't cut too deeply. Cut/sparkle is most important to me.
That said, being Cartier, I'm sure the stones they choose are great. For the neck, and with the options presented, I would choose the 0.80 F IF 15.400€ fluorescence or 0.87 F VS2 13.000€ fluorescence. If there is not much difference between them when you view them, I would go for the 0.87 F VS2 13.000€ fluorescence to save/use the funds for other things.